This invention relates to organic and inorganic anion-pillared clay compositions having the hydrotalcite crystal structure and, more particularly, to anionic magnesium-aluminum hydrotalcite clays having large inorganic and/or organic anions located interstitially between positively charged layers of metal hydroxides and a method of preparation of such anion-pillared clays to form anionic clays where the anion is at least in part a polyoxometalate anion.
Clay minerals are composed of infinite layers of metal or nonmetal oxides and hydroxides stacked one on top of the other. In the case of the widely-found cationic clays, interlayer cations (Na.sup.+, Ca.sup.2+ etc.) charge neutralize the negatively charged oxide/hydroxide sheets. The far less common anionic clays have positively charged metal oxide/hydroxide layers with anions located interstitially. Many of these are based on double hydroxides of such main group metals as Mg, and Al and transition metals such as Ni, Co, Cr, Zn and Fe etc. These clays have a structure similar to brucite [Mg(OH).sub.2 ] in which the magnesium ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In the anionic clays, some of the magnesium is isomorphously replaced by a trivalent ion, say Al.sup.3+. The Mg.sup.2+, Al.sup.3+, OH.sup.- layers are then positively charged necessitating charge balancing by insertion of anions between the layers. One such clay is hydrotalcite in which the carbonate ion is the interstitial anion. Rhombohedral hydrotalcite has the idealized unit cell formula [Mg.sub.6 Al.sub.2 (OH).sub.16 ]CO.sub.3.4H.sub.2 O. However, the ratio of Mg/Al in hydrotalcite can vary between 1.7 and 4 and various other divalent and trivalent ions may be substituted for the magnesium and aluminum. In addition, the anion, which is carbonate in hydrotalcite, can be varied in synthesis by a large number of simple anions such as NO.sub.3.sup.-, Cl.sup.-, OH.sup.-, SO.sub.4.sup.2- etc. Substitution techniques which have been used are ion exchange and acid treatment in the presence of the desired anion.
Processes for making hydrotalcite clay have been the subject of a number of patents. See, for example, U.S. Pat. Nos. 4,539,306, 4,539,195 and 3,539,306. These patents are largely directed to pharmaceutical uses for hydrotalcite. Miyata et al. in U.S. Pat. Nos. 3,796,792, 3,879,523, and 3,879,525 describes hydrotalcites with both cationic layer and anionic substitution including the transition metal anions CrO.sub.4.sup.2-, MoO.sub.4.sup.2- and Mo.sub.2 O.sub.7.sup.2-. Both compositions and preparative methods are described, and the compositions are said to be useful for catalytic purposes, absorbents, dessicants and the like. In U.S. Pat. Nos. 4,458,026 and 476,324 and Journal of Catalysis 94, 547-57 (1985), Reichle describes synthetic hydrotalcites containing small anions, including anions of some transition elements, and also large organic anions such as long chain aliphatic alpha-omega dicarboxylates. However, no X-ray data is given to support his assumption that the carboxylate anion is in the lattice. Reichle has shown that the hydrotalcites can catalyze aldol condensations effectively. Miyata and Kumura in Chemistry Letters pp. 843-8 (1973) describe hydrotalcite clay materials containing Zn.sup.2+ and Al.sup.3+ with dicarboxylate anions and show that the interlayer spacing obtained using X-ray diffraction expands from 9.4 Angstroms to about 19 Angstroms as the dicarboxylate anion is changed along the series oxalate, malonate, succinate, adipate and sebacate. This study indicates the carboxylate anions are in the lattice standing roughly perpendicular to the layers.
The success of molecular sieves for catalytic purposes has prompted a search for other porous inorganic materials which could act as shape selective catalysts. Pillared cationic clays have been investigated as part of this search but the small number of useful large cations and the small amount of open volume left after completely pillaring some of the materials has been a concern. Also, the poor thermal stability of several of the cationic pillars has been discouraging since the pillars collapse during higher temperature catalytic use. Anionic clays have also been considered, as has been reviewed above, but the work has resulted in no examples of anionic clays with catalytic potential i.e., clays with a large interlayer spacing, an incompletely stuffed gallery, and supported by inorganic anionic pillars containing a transition metal ion. Now a useful technique has been found to produce large organic anion pillared Mg/Al hydrotalcites with large interlayer spacings which can be exchanged with large transition-metal-containing anions to form Mg/Al hydrotalcites containing those inorganic ions. Partial replacement with such large inorganic ions can produce structures in which some of the hydroxide layer charge is compensated by smaller anions increasing the gallery space available to molecules diffusing into the structure from the outside and the potential for catalytic action.